The block transmission of information symbols through a distorting channel rapidly variable over time, such as a frequency-selective fading radio-frequency channel, imposes severe constraints at the receiving end, so as to allow suitable reception and suitable decoding of these symbols thus transmitted. Information symbols are understood as any signal consisting either of the values of logic variables or the values of digitised analogue variables.
In particular, reception can only be performed in the presence of inter-symbol interference. Such a system for block transmission has been described in the article by F. Hsu, entitled "Data Directed Estimation Techniques for Single-Tone HF Modems", IEEE Military Commun. Conf., Boston, Mass., Oct. 1985, pp. 12.4.1-10. This system has been recognised as appropriate and has been adopted in the standards describing and fixing the technical characteristics of modems for the (HF) ionospheric channel and may also be suitable in systems for communicating with mobiles.
By way of illustration, it is recalled that this system for block transmission is based on the assumption by which the transmission channel is constant, in its physical or radio-frequency characteristics, during the transmission of a sufficiently short symbol block. Estimation of the channel, or of the physical or radio-frequency parameters of the latter, which estimation is required for the processing performed at the receiving end, is obtained by virtue of the transmission of known symbols, in the form of blocks BI interleaved between the information symbol blocks, BE, as illustrated in FIG. 1a. These known symbol blocks BI also have the role of subtracting their interference from the processed signal and of thus separating the symbol blocks BE so that each block of successive symbols may be processed independently of the adjacent blocks. Such a measure implies that the number of symbols of a known block is not less than the duration of the impulse response of the channel, expressed in number of symbols, more commonly designated the channel memory. A corresponding appropriate receiver is also described in the aforesaid article and is designated the "Nonlinear Decision-Directed Estimator" or NDDE. This type of receiver in fact requires the solution of M/2 systems of equations of decreasing dimensions, M/2, . . . , 2, for symbol blocks containing M symbols.
FIG. 1b shows a part of a baseband equivalent model of such a communication system. The symbols dn, in complex notation, are independent and identically distributed, and take equiprobable values in a finite alphabet of symbols. The symbols modulate the amplitude of a finite energy pulse f(t) with support [0,LT] where T represents the duration which separates the dispatching of two consecutive symbols. The noise generated during transmission is centred, Gaussian noise with power spectral density 2N.sub.0.
Reception begins with optimal matched filtering f*(-t), the asterisk "*" designating the complex conjugate, followed by sampling, at the symbol rate. Assuming that the channel, that is to say its physical and radio-frequency parameters, is constant during the observation of a symbol block and that a perfect estimate of f(t) is then available at the receiver, a numerical implementation of the matched filtering can be carried out. The output from the matched filter sampled at the instants nT is then written: ##EQU1## a relation in which {rk} designates the samples at the instants kT of the impulse response of the cascade consisting of the finite energy pulse f(t) and of the matched filter f*(-t). Note the Hermitian symmetry r-k=rk*. The samples uk, consisting in fact of the contribution due to the noise during the sampling at the receiving end, are the realisations of centred Gaussian random variables with auto-correlation function EQU E(un.un-k*)=2N.sub.0.rk.
In such a system for block transmission, the interference of the known symbols dn of an interleaf block BI, with n&gt;M and n&lt;1, which neighbour a symbol block BE composed of symbols dn with n.epsilon.[1,M] is subtracted from the sample y'n obtained at the output of the matched filter, thus making it possible to obtain an observation vector EQU y=(y.sub.M, y.sub.M-1, . . . , y.sub.1).sup.t
which does not depend on the known samples but on the following symbol vector EQU D=(d.sub.M, d.sub.M-1, . . . , d.sub.1).sup.t
where t designates the transposition operator. Every observation vector Y satisfies the relation: EQU Y=R.D+U
a relation in which R is a Toeplitz matrix of dimension M.times.M, with Hermitian symmetry and with elements EQU R.sub.ij =rj-1 with i,j=0,1, . . . , M-1.
U is a realisation of a centred Gaussian vector with covariance matrix 2N.sub.0.R.
The receivers of this type, designated NDDE receivers, make it possible to obtain, from the vector Y, a reliable decision regarding the despatched symbol vector D. However, this type of receiver necessitates the solution of M/2 systems of equations of decreasing dimensions M/2,M/2, . . . , 2 where M, the number of symbols per block, is assumed to be even.
Finally, equalisers designated DFE equalisers, Decision Feedback Equalisers, are normally adapted for continuous transmission. Their use for block transmission is therefore less efficacious.